Conventionally, an ultrasonic flaw detection test is executed to nondestructively inspect the depth of a flaw in a surface crack of a weld of a thick pipe such as a weld of a pipe of an atomic power plant and the depth and the position of a flaw including a corroded portion near a surface layer and a minute flaw, the height of a flaw from a back surface, and the like. In the nondestructive inspection to the weld of the thick pipe, needs for accurately measuring the height, the depth, the size of a flaw and the like are increased in addition to simply detecting whether or not a flaw is present. In the sizing of a defect, since it is indispensable to detect a flaw generation edge and a flaw termination edge with a pinpoint accuracy, it is recently contemplated to apply ultrasonic flaw detection methods such as a phased array method, a TOFD method, i.e., Time of Flight Diffraction method, an angle flaw detection method, and the like.
For example, the TOFD method making use of a surface wave is generally used to estimate the depth of a flaw of a surface crack (Non-Patent Document 1). As shown in FIG. 13, according to the TOFD method, the difference in paths is generated between the beam path T0 of a surface wave when a crack is not present and the beam path T1 of the surface wave when a crack is present. Thus, after the beam path T0 of the surface wave when the crack is not present is previously determined and the beam path T1 of the surface wave when the crack is present is determined, a flaw depth d is estimated from the relation shown in Expression 1. The beam path means a distance in which a beam passes between a probe on a transmission side and a probe on a reception side.d=½(T1−T0)  (1)
Further, the flaw height may be measured by an angle flaw detection method using an angle probe (Non-Patent Document 2).
Further, the inventors have proposed an ultrasonic measuring method called an SPOD method, i.e., Short Path of Diffraction method, which is suitable for measuring a flaw in the periphery of a weld of austenite-based stainless steel and Inconel (registered trademark of Special Metals Corporation) which is mainly used in an in-furnace structure, a circulating piping, and the like of a primary structure of an atomic power plant, in particular, the height of a flaw in a back surface opening (Non-Patent Document 3). In the SPOD method, a flaw height is determined from the difference between the arriving times of a component, which directly propagates above a flaw, and a component which propagates above the flaw after it is reflected on the back surface of a specimen in such a manner that an angle probe is combined with an a normal probe, an ultrasonic pulse is caused to be incident on a flaw in the specimen from an oblique direction, and a diffracted wave generated at an edge of the flaw is received by the normal probe above the flaw.
Non-Patent Document 1: The Japanese Society for Non-Destructive Inspection, “Flaw Height Measuring Method by TOFD Method Standardized by The Japanese Society for Non-Destructive Inspection”, published on Dec. 1, 2001
Non-Patent Document 2: The Japanese Society for Non-Destructive Inspection, “Flaw Height Measuring Method by Tip Echo Techniques Standardized by The Japanese Society for Non-Destructive Inspection”, published on Jun. 1, 1997
Non-Patent Document 3: “Proposal of Simple Flaw Sizing Method in Ultrasonic Flaw Detection Test”, Program & Abstracts of Second Academic Lecture of Conservation Society of Japan, pp 21-26, distributed on Jul. 8, 2005